Principles of Spatial Filters
Figure 1 - Spatial Filter System
The input Gaussian beam has added to it spatially varying intensity "noise." When a beam is focused by an aspheric lens, the input beam is transformed into a central Gaussian spot (on the optical axis) and side fringes, which represent the unwanted "noise" (see Figure 2 below). The radial position of the side fringes is proportional to the spatial frequency of the "noise".
By centering a pinhole on a central Gaussian spot, the "clean" portion of the beam can pass while the "noise" fringes are blocked (see Figure 3 below).
The diffraction-limited spot size at the 99% contour is given by:
where λ = wavelength, ƒ=focal length and r = input beam 1/e2 radius.
A pinhole that is approximately 30% larger than the diffraction-limited spot size should be chosen to allow the focused Gaussian spot to pass while blocking the "noise" fringes that are shifted off axis. If the pinhole is too small, the beam will be clipped, but if it is too large, additional noise will get through the pinhole.
For more information on creating a spatial filter system for your application, please see the Tutorial tab on the Spatial Filters Systems page.